1. Field of the Invention
The present invention relates generally to DC/DC converters and more particularly to charge pump converters.
2. Description of the Related Art
Charge pumps are DC/DC converters characterized by a pump capacitor and a reservoir capacitor embedded in a switch network. The reservoir capacitor typically provides voltage and current to an external load circuit. The switch network is typically configured to conduct current between a supply voltage and the pump capacitor and subsequently conduct charge between the pump capacitor and the reservoir capacitor. The specific switch network configuration determines the amplitude and polarity of the reservoir voltage presented to the load circuit.
One specific switch configuration is illustrated in the FIGS. 1A and 1B schematics of a prior art charge pump inverter 20. These schematics show the inverter 20 in first and second operational phases. The circuit structure of the voltage inverter 20 includes switches 22, 24 and capacitor 26 arranged in series between an inverter input 30 and an inverter output 32. The switch 22 adjoins the input 30 and the capacitor 26 is disposed between the two switches. Switches 36, 38 and capacitor 40 each have one terminal connected to a ground 42 with the other switch terminals attached to opposite sides of the capacitor 26 and the other capacitor 40 terminal in common with the inverter output 32. The capacitors 26, 40 are often respectively termed the pump capacitor and the reservoir capacitor and the input 30 is typically attached to a voltage supply V+.
In operation, the switches of the inverter 20 are opened and closed in two phases. In the first phase illustrated in FIG. 1A, switches 22 and 38 are closed and switches 24 and 36 are open. In the second phase illustrated in FIG. 1B, switches 22 and 38 are open and switches 24 and 36 are closed.
In FIG. 1A, the voltage on reservoir capacitor 40 is supplied to any circuit attached to the inverter output 32 and the capacitor is isolated from the remainder of the circuit. In this phase, the voltage V+ at the inverter input 30 supplies charge to the capacitor 26 until it develops a voltage V+ across it with the positive potential on the capacitor lead 44. When the switches 22, 24, 36 and 38 are changed to the second phase positions of FIG. 1B, the capacitor lead 44 is connected to the ground 42 so that, with respect to ground, a V- potential is established at the capacitor lead 46. The capacitors 26, 40 are now in parallel and the lead 46 is in common with the output 32. During this second phase, the capacitor 26 pumps charge into the reservoir capacitor 40 causing its potential to approach V-.
It is apparent, therefore, that the reservoir capacitor 40 has a voltage of approximately V- at the beginning of each first phase illustrated in FIG. 1A and the amplitude of this voltage decays during each first phase in accordance with the current demand of the circuit attached to the output 32. During the second phase illustrated in FIG. 1B, the pump capacitor 26 pumps charge into the reservoir capacitor 40 causing its voltage to again approach V-. In a similar manner, the voltage amplitude across capacitor 26 decays during the second phase as it pumps the reservoir capacitor 40 and then rises again to V- during the first phase. Thus, the voltage across both capacitors 26, 40 exhibits a ripple whose amplitude is a function of the load current and the size of the capacitors. As a consequence of the operational phases described above, a circuit connected to the output 32 is supplied with a voltage that is opposite and approximately equal to the voltage supply at the input 30.
Another specific switch configuration is illustrated in the FIGS. 2A and 2B schematics of a prior art charge pump doubler 60. Similar to the FIGS. 1A and 1B schematics, these doubler schematics show first and second operational phases. The charge pump doubler 60 includes switches 62, 64 arranged in series between a doubler input 70 and a doubler output 72. Another pair of switches 76, 78 are arranged in series and disposed between the input 70 and a ground 82. Switches 62 and 76 adjoin the input 70. A capacitor 84 has one terminal connected to the junction of switches 62, 64 and the other to the junction of switches 76, 78. A capacitor 86 is disposed between the output 72 and ground 82. The input 70 is connected to a voltage supply V+. In this circuit structure, the pump and reservoir terms can be respectively applied to the capacitor 80 and the capacitor 86.
In the first operational phase of FIG. 2A, the switches 62, 78 are closed and the switches 64, 76 are open. In the second phase shown in FIG. 2B, switches 62, 78 are open and switches 64, 76 are closed. In FIG. 2A, the voltage on capacitor 86 is supplied to the load circuit attached to the doubler output 72 while the voltage V+ at the doubler input 70 delivers charge to the capacitor 84. A voltage V+ is developed across the capacitor 84 with a positive potential on the capacitor lead 88 relative to the opposite lead 90.
When the switches are changed to the second phase positions of FIG. 2B, the capacitor leads 90, 88 are respectively connected to the input 70 and the output 72. The potential on the capacitor 84 now adds to the input voltage V+ to place a voltage 2 V+ at the output 72. During this second phase, the capacitor 84 pumps charge into the reservoir capacitor 86 causing its potential to approach 2 V+.
Therefore, the reservoir capacitor 86 has a voltage of approximately 2 V+ at the beginning of each first phase illustrated in FIG. 2A. This voltage amplitude decays during each first phase in accordance with the current demand of the circuit attached to the output 72. During the second phase illustrated in FIG. 2B, the pump capacitor 84 pumps charge into the reservoir capacitor 86 causing its voltage to again approach 2 V+. Similarly, the voltage across capacitor 84 decays during the second phase as it pumps the reservoir capacitor 86 and then rises again to 2 V during the first phase. As a consequence of the operation of the doubler circuit 60, a circuit connected to the output 72 is supplied with a voltage approximately twice the amplitude of the voltage supply at the input 70.
Although the circuits 20, 60 illustrated in FIGS. 1A-2B are shown to have a source voltage V+ attached to their inputs, it should be apparent that nonpolarized capacitors allow them to operate with a source voltage of either polarity. In practice, the two described phases of these circuits are typically separated by a comparatively short transitional phase to accommodate circuit switching times and protect circuit elements. For example, the voltage source V+ at the circuit input 30 of FIG. 1A would be shorted to ground if switch 36 moved to its FIG. 1B position prior to the same move by switch 22. Thus, in the transitional phase, switch 22 is first opened and switch 36 is subsequently closed.
Other exemplary switch network configurations are directed to the generation of different output voltages as a function of the source voltage, e.g., a selectable bipolar doubled output voltage. U.S. Patent directed to charge pump structures include 4,636,930; 4,679,134; 4,777,577; 4,797,899; 4,809,152; 4,897,774 and 5,237,209.